Static pressure correction system



Sept. 5, 1967 R. w. ARMSTRONG STATIC PRESSURE CORRECTION SYSTEM 2Sheets-Sheet 2 Original Filed April 27 [F 0. INVENTOR. ROBERT W.ARMSTRONG ATTORNEY.

United States Patent 3,339,408 STATIC PRESSURE CORRECTION SYSTEM RobertW. Armstrong, Mound, Minn., assiguor to Honeywell Inc., a corporation ofDelaware Original application Apr. 27, 1962, Ser. No. 191,685, nowPatent No. 3,239,140, dated Mar. 8, 1966. Divided and this applicationApr. 5, 1965, Ser. No. 452,964

3 Claims. (Cl. 73-178) This application is a division of a copendingapplication, filed Apr. 27, 1962, Ser. No. 191,685, now Patent No.3,239,140 of the present inventor and assigned to the assignee of thepresent invention.

This invention relates to control apparatus and more particularly to airdata computer apparatus utilizing novel static error correctionapparatus to convert signals indicative of indicated static pressure (Pto true static pressure (P and provide corrected outputs for thoseaircraft components that rely on true static pressure for properindication.

It has long been known in the art that the static pressure received bythe sensing device or Pitot-static tube mounted on an aircraft becomesincorrect by an amount AP which varies as a function of aircraft speedin terms of Mach number M. A relationship may be expressed as:

In the past a number of systems have been proposed to provide acorrected static pressure source but all of these systems have had anumber of disadvantages. Pneumatic systems have been proposed whichemploy a pump or other pressure regulating device controlled inaccordance with the static error pressure AP to adjust the staticpressure in such a manner that the pneumatic output thereof is correctedstatic pressure P These systems have had the major disadvantage ofslowresponse and for the most part are undesirable in the present day highspeed aircraft where fast response is critical. Other proposed systemsinclude electrically rebalanced systems wherein an electrical signal isgenerated indicative of corrected static pressure. Such electricalsystems have had the major disadvantage that rthe present state of theart is incapable of manufacturing accurate enough components for thesesystems. A number of problems result such as undesirable quadraturesignals, impedance matching and the like which also make such systemsundesirable.

The present invention provides an output indicative of corrected staticpressure which is mechanical in nature and as such is not subjected tothe slow response of pneumatic systems nor to the undesirable feature ofelectric systems. Briefly the present invention includes a pressuretransducer having indicated static pressure P as one input and providingan out-put in the form of a shaft rotation in accordance therewith. Amechanical static pressure error correcting device is provided withreceives as an input the shaft rotation produced by the transducerdevice and also receives as a second input a shaft rotation which ischaracterized as a function of airspeed in terms of Mach number. Themechanical static error correcting device operates upon these two inputsto provide an output indicative of the static pressure error AP whichoutput is also in the form of a shaft rotation. This AP signal ismechanically coupled back to the transducing apparatus to adjust theapparatus in such a direction that the correction signal AP is combinedwith the signal indicative of indicated static pressure P so that theoutput of the transducer becomes corrected static pressure P The outputof the transducer which is now corrected static pressure P may be usedin the various portions of the aircraft instruments where correctedstatic pressure is needed and no further correction is necessary. Theoutput of the static error cor- Patented Sept. 5 1967 lCe rector, asstated, is the correction signal AP and this in itself may be used tocorrect other instruments such as the differential pressure transducer.

A more complete understanding of the present invention will be obtainedupon examination of the following specification, claims and drawings inwhich:

FIGURE 1 is a schematic representation of the basic principle involvedin the static error correction mechanism;

FIGURE 2 is a partly schematic and partly exploded view of the staticpressure transducer and static error correcting mechanism employing thepresent invention; and

FIGURE '3 is a block diagram of an air data computing system employingstatic error correction.

Referring now to FIGURE 1, a rotatable arm 10 is shown having alaterally extending flange 12 at one end thereof which contains a pivotaxis 14. The rotatable member 10 has a bearing surface 16 which extendsunder the flange portion 12 as shown by the dashed line 18.

Also shown in FIGURE 1 is a movable member 20 having a flange portion 22upon which is mounted a bearing member or roller 24. Roller 24 is causedto bear against the bearing surface 16 of member 10 and the pivot axis26 of roller 24 is such that it is the same distance from the bearingsurface 16 as the pivot axis 14 of member 10. By this arrangement it isseen that member 20 can be placed so that roller 24 is positioneddirectly under the flange portion 12 of member 10 so that axis 26 ofroller 24 would coincide with axis 14 of member 10.

Member 20 is caused by means not shown to move in a vertical directionunder the influence of static pressure and under zero static pressureconditions axis 26 would coincide with axis 14. The verticaldisplacement of axis 26 from coincidence with axis 14 will be controlledaccording to static pressure and in FIGURE 1 this distance has beenshown as the vertical line of a right triangle identified with referencenumeral P Member 10 is caused to rotate as a function of aircraft speedin terms of Mach number. It is known that the static pressure error APvaries with Mach number in a predetermined fashion and consequently theratio AP/P is also a function of airspeed in terms of Mach number. Bycharacterized means not shown in FIGURE 1, member 10 is caused to rotateas a function of arc tangent AP/P and the amount of rotation has beenshown in FIGURE 1 asangle 0 which is the angle between the vertical sideof the triangle P and the line joining axis 14 with axis 26 whichcomprises the hypotenuse of the triangle shown in FIGURE 1.

The amount of horizontal movement of axis 26 from the zero staticpressure condition has been shown in FIG- URE 1 as the distance of thehorizontal portion of the triangle identified by reference numeral X. Itis clear from FIGURE 1 and the triangle that-X=P tangent B'and since, aspreviously stated, 0 is caused to be a function of arc tangent AP/P X=KPa tangent (are tangent g AP (P.

or X =KAP, where K is a factor of proportionality determined by thegeometry of the mechanism.

It is therefore seen that the amount of horizontal displacement of pivotpoint 26 and consequently the horizontal displacement of member 20 isproportional to the static error signal AP and means are provided notshown in FIGURE 1 to measure this horizontal displacement and convert itinto the AP signal used for the remainder of the system.

FIGURE 2 shows a practical embodiment of the present invention includingin the lower portion the static pressure transducer identified byreference numeral 50 and in the upper portion the static errorcorrection mechanism identified by reference numeral 52.

The static pressure transducer comprises a Z-shaped member 55 which isfixed to the container (not shown) which houses the transducermechanism. Z-shaped member 55 has two oppositely directed flanges 57 and58 at either end thereof which flanges have connected thereto a pair ofpressure sensitive devices or bellows 61 and 62 respectively. Bellows 61and 62 are both evacuated in the static pressure transducer 50 while theinterior of the container is supplied with the indicated static pressureP provided from the Pitot-static probe on the aircraft. Thus as theindicated static pressure changes belows 61 and 62 will each contract orexpand. Since bellows 61 and 62 are oppositely directed, they produceforces with changes in indicated static pressure in opposite directions.Bellows 61 and 62 each have a force applying linkage shown in FIGURE 2as bars 66 and 67 respectively. Bars 66 and 67 are connected on oppositesides of a generally O-shaped yoke 70. Yoke 70 is mounted on theZ-shaped member 55 by a quadrilever type spring 72 which is fastened tothe Z-shaped member 55 at the center and which extends in oppositedirections to the yoke 70 where it is fastened as at 75 and 76respectively. This quadrilever type spring 72 provides a spring biasedpivot for the yoke 70 and since bellows 61 and 62 apply forces varyingwith P to yoke member 70 in opposite directions, changes in P produce atorque on yoke member 70 which turns yoke member 70 with respect to theZ member 55 by an amount depending upon the stiffness of the transducersystem spring rate and the amount of change of P Yoke member 70 has alateral extension 80 which is in the form of a magnetic armaturecooperting with an E-shaped transformer 83 which is connected to thecontainer housing the transducer 50 by means not shown. It is seenhowever that rotation of yoke 70 causes displacement of armature 80 withrespect to E transformer 83 whenever the indicated static pressure Pchanges. This displacement of the armature 80 with respect to the Etransformer 83 produces a signal on wires 85, 86 and 87 which lead to amotor shown generally as 90. Thus any change in indicated staticpressure P will cause motor 90 to operate in a first or second directiondepending upon whether P has increased or decreased. Of course, othertype pickolf means may be employed in place of E transformer 83 and ifnecessary an amplifier may be used between the pickoff and the motor 90.

Motor 90 has an ouput shown by dashed line 95 which is shown in FIGURE 2terminating at reference numeral A. The dashed line connection 95 isconnected by means not shown in FIGURE 2 to a corresponding referencenumeral A, shown in the upper portion of the drawing, which is connectedby mechanical connection shown as dashed line 96 to a gear sector 98which forms a portion of a torsion tube 100. As will be described withregard to FIGURE 3, various apparatus may be included between thereference numerals A and A such as gear trains and earns but are notshown in FIGURE 2 for purposes of clarity. Torsion tube 100 has a clamp102 at the upper end thereof which clamps a torsion bar 104. Torsion bar104 is shown generally rectangular in cross section and extends downthrough the torsion tube 100 and down through the center of thequadrilever spring 72 and is clamped to the yoke member 70 at the bottomportion thereof by a clamp 108. It is seen that rotation of motor 90operating through mechanical connections 95 and 96 will cause rotationof gear sector 98 and torsion tube 100 to apply a torque to the torsionbar 104 which in turn transmits this torque to the yoke member 70.Rotation of yoke member 70 occasioned by a change in indicated staticpressure P will thus cause movement of armature 80 with respect to atransformer 83 and drive motor 90 in such a direction that the rotationof torsion tube and torsion bar 104 applies an oppositely directedtorque to the yoke member 70 to accomplish rebalance of the system. Thatis to say, as soon as yoke member 70 rotates under the influence of achange in indicated static pressure P an oppositely directed torque isapplied by means of torsion bar 104 to yoke member 70 to return it toits original state of equilibrium at which time motor 90 stops and theposition thereof would be indicative of the new indicated staticpressure P Also connected to the output of motor 90 is a mechanicalconnection shown as dashed line 112 which operates to turn a gear 115 inthe static error correction mechanism 52. Gear 115 cooperates with arack 116 which is part of a member 117 in the static error correctormechanism 52. Member 117 is provided with guide means such as a trough120 in which a pair of rollers 123 and 124 operate. Rollers 123 and 124are connected to the frame or casing (not shown) housing the staticerror correction mechanism 52. Member 117 also has a lateral extensionin the form of a flat plate which extends to the left in FIGURE 2 andwhich has a vertically extending abutment 133 at the remote end thereof.Vertically extending abutment 133 is also provided with guide means suchas a trough 135 in which members such as roller 137 operate. Roller 137is likewise connected to the frame or casing (not shown) housing thestatic error correction mechanism 52. Rollers 123, 124 and 137 operateto guide member 117 and the lateral extension 130 in a directiongenerally into and out of the plane of the drawing of FIGURE 2. As seen,enerigization of motor 90 will cause rotation of gear 115 which will inturn cause motion of member 117 as guided by rollers 123, 124 and 137.

Member 117 carries a pair of rollers or guide members 140 and 141 andalso carries on the vertically extending abutment 133 a pair of rollersor guide members 144 and 145. Rollers 140, 141, 144 and 145 on member117 cooperate with a generally T-shaped member and operate to guidemember 150 in a direction perpendicular to the motion of member 117.That is to say, T-shaped member 150 is able to move in a directionsubstantially parallel to the plane of the drawing of FIGURE 2.

Member 150 has a lateral extension 153 which in turn carries a guidemember such as a roller 155 thereon. Roller 155 is positioned so as tobear on a surface of a rotatable member 162.

Rotatable member 162 is shown in FIGURE 2 as formed like an open boxthrough which member 150 extends in such a manner that roller 155 cancooperate with the surface 160. Member 162 also has a laterallyextending portion 165 which carries a pivot 166. Pivot 166 is positionedon extension 165 at a distance from surface 160, which extends underextension 165, equal to the distance from the pivot of roller 155 to thesurface 160. This enables roller 155 to occupy a position whereby itspivot is directly under pivot 166. Pivot 166 is connected by hearingmeans (not shown) to the frame or casing housing the static errorcorrection mechanism 52 so that it is free to rotate with respectthereto. Connected to the upper portion of pivot 166 is a follower arm172 which extends to a cam 174 and which has a roller or follower means175 cooperating with the surface of cam 174. Cam 174 in turn has a shaftconnected thereto which is caused to rotate by means not shown in FIG-URE 2 as a function of airspeed in terms of Mach number as indicated bythe arrow and the designation M in FIGURE 2.

In the apparatus thus far described it is seen that rotation of shaft180 will cause rotation of cam 174 and will thus cause movement offollower arm 172 which operating through pivot 166 will cause rotatablemember 162 to rotate. This rotation will be accompanied by movement ofthe roller 155 to the left or to the right in FIGURE 2 depending uponthe direction of rotation of member 162. Likewise it is seen thatrotation of shaft 112 connected to motor 90 will cause rotation of gear115 and thus cause movement of member 117 generally into or out of theplane of the drawing of FIGURE 2. This movement of member 117 will beaccompanied by movement of member 150 to the right or to the left sinceroller 155 is caused to bear against surface 160.

As explained with regard to FIGURE 1, motion of member 150 to the leftor to the right is a product of the amount of movement of member 117 andthe tangent of the angle through which rotatable member 162 has rotated.As stated, the motion of member 117 is proportional to static pressureas provided by the motor 90 and rotation of member 162 is proportionalto a desired function of Mach number. Cam 174 is so characterized thatrotation of member 162 is proportional to the arc tangent of the ratioAP/P which ratio is known to be a predetermined function of aircraftspeed in terms of Mach number. Thus, motion of member 150 to the leftand to the right is indicative of the correction factor AP since themechanism operates to multiply the ratio This motion to the right or tothe left indicative of AP is transmitted from member 150 by means of aT-shape-d extension 190 at the left end thereof which cooperates with aguide member or roller 192 connected to a rack 194. Thus motion ofmember 150 will be accompanied by motion of rack 194 and this motion istransmitted to a gear 196 which is mounted on a shaft 198. It is seenthat rotation of shaft 198 is also proportional to the static errorcorrection AP. Spring means shown as arrow 200 operate on rack 194 toforce roller 192 against the T- shaped surface 190 and thus push member150 and roller XPB 155 against the surface 160 of rotatable member 162.

Spring 200 therefore holds the mechanism in an engaged position with thevarious rollers and surfaces biased towards each other.

As stated shaft 198 rotates in accordance with the static errorcorrection AP and this rotation is imparted to a gear sector 204connected at the lower end thereof.

Gear sector 204 cooperates With a gear sector 208 which forms a part ofa member 210. Member 210 has first and second elongated extensions 212and 214 respectively which are attached to a pair of cantilever springmembers 216 and 218 as shown by dashed lines 220 and 222. Springs 216and 218 are fastened to the lower portion of yoke member 70 on oppositesides and spaced from clamp 108 by means of a pair of extensions such asshown at 228. It is seen that as gear sector 204 rotates member 210 willrotate which will in turn apply a torque to yoke member 70 throughsprings 216 and 218. Since the rotation of gear sector 204 isproportional to that of shaft 198 which in turn is proportional to thestatic error correction quantity AP, the correcting force applied frommember 210 through springs 216 and 218 to yoke 70 is proportional to thestatic error correction AP. This correction torque on yoke member 70causes movement thereof in a direction which when taken together withthe torque applied from the indicated static pressure P provides thenecessary correction and the re sultant movement of yoke-member 70 isproportional to corrected static pressure P Stated differently, themechanism shown in the force transducer 50 applies a correcting force APwhich is algebraically summed with the indicated static pressure force Pto provide anoutput P which as mentioned is related to corrected staticpressure P by the equation P P =AP. Thus it is seen that the pressuretransducer 50 has an output proportional to corrected static pressure Pwhich output may be utilized in the aircraft for those indications wherecorrected static pressure is necessary.

Also shown in FIGURE 2 is a member 250 which has a pair of upwardlyextending cylindrical portions 252 and 253. Portions 252 and 2530fmember 250 when the unit is assembled are attached to the Z-shapedmember 55 at holes 260 and 261 respectively as shown by dashed hnes 264and 265. Member 250 has a bearing connection with member 210 at theportion between them shown by arrow 270 so that member 210 is free torotate with respect to member 250.

Also shown in FIGURE 2 is a force applying device 275 attached to theyoke member at point 277. Member 275 carries a temperature sensitiveelement 279 which operates to apply a force to the yoke member 70 inaccordance with temperature and thereby provide the transducer 50 withtemperature compensation.

Also shown in FIGURE 2 is a transducer 300 identified with the letter qTransducer 300 is substantially identical in structure to staticpressure transducer 50 herein described and will not be itself describedin detail. The only difference between the q transducer 300 and thestatic pressure transducer 50 is that the q transducer produces anoutput proportional to the difference between total pressure P andstatic pressure P This difference is defined as q and requires that thetransducer have two inputs P as shown by conduit 303 and P as shown byconduit 304. The interior of transducer 300 will be the same as theinterior of the static pressure transducer 50 herein described exceptthat the bellows 61 and 62 which were described as evacuated in thestatic pressure transducer 50 will have static pressure applied to theirinteriors and total pressure P in the case will be applied to theirexteriors. Therefore in the q transducer 300 motion of the yoke membercorresponding to member 70 will be in accordance with the differencebetween total pressure P and static pressure P Transducer 300 has aninput correcting for the static error AP by means of a shaft 308 whichcorresponds to the shaft 198 for the static pressure transducer 50.Shaft 308 is connected to a gear 310 which is in turn connected to agear 312 mounted on the shaft 198. Therefore as the rack member 194moves in accordance with static error correction AP, gear 312, gear 310and shaft 308 will rotate accordingly. By a mechanism similar to thatshown in describing the static pressure transducer 50, this correctionfactor AP will be applied to the q transducer so that its output isindicative of the difference between total pressure P and correctedstatic pressure P Referring now to FIGURE 3, which shows in schematicdiagram form one embodiment of an air data computer incorporating thestatic error correction, a P transducer 50, which may be the same asthat apparatus shown in FIGURE 2, is shown having an input conduit 400which provides the transducer with the pressure P derived from theindicated static pressure source on the aircraft. A mechanical inputshown as dash line 402 is shown connected to transducer 50 to supply thetransducer with the correction signal AI. Mechanical connection 402 isshown connected by a second mechanical connection 405 to the staticerror corrector 52 which may be the same as that shown in FIG. 2.Mechanical connections 402 and 405 may comprise the apparatus includingroller 192, rack 194, gear 196 and shaft 198 of FIGURE 2.

Static pressure transducer 50 provides an electrical output whenever achange in true static pressure P causes unbalance of transducer 50. Thisoutput is shown in FIGURE 3 as emerging on conductor 407 and being fedto a summing network 409 which is in turn connected by a conductor 411to an amplifier 412. The output of amplifier 412 drives motor by meansof a connection 415. Motor 90 in FIGURE 3 is the equivalent of motor 90in FIGURE 2 and the conductor 407, summing network 409, conductor 411,amplifier 412 and connection 415 comprises apparatus placed between Etrans-former 83 and motor 90 which in FIGURE 2 was shown merely asconductors 85, '86 and 87.

Rate feedback is provided in FIGURE 3 by means of a velocitygenerator'420'connected to motor 90 by a mechanical connection shown asdash line 421. The output of velocity generator 420 is fed to a resistor423 by a conductor 424. Resistor 423 includes a movable wiper 425 whichis connected back to the summing network 409 to provide the desired ratefeedback.

The mechanical motion of motor 90 is caused to rebalance the Ptransducer 50 in the following manner. A mechanical connection 427 isplaced between motor 90 and a gear train 429. Gear train 429 has a firstoutput connection 431 leading to a second gear train 432. Gear train 432has mechanical connection 433 leading to a characterized cam shown inFIGURE 3 as a tape cam 435. Cam 435 is so characterized that it convertsmotion indicative of a first condition to a motion indicative of the logof the first condition or vice-versa, Specifically this cam is socharacterized as to convert log of P to P Thus if a first portion 437 ofcam 435 turns as the log of P a second portion 438 of cam 435 will turnas P Cam 435 has a mechanical connection shown as dash line 440connected to a second mechanical connection shown as dash line 442 to agear train 443 and from there by a mechanical connection shown as dashline 444 to the P transducer 50 to accomplish rebalance. Connection 442is also shown providing an input to the static error correctionmechanism 52. The mechanical connections above described between motor90 and P transducer 50 including elements 427 through 444 may compriseapparatus between A and A in FIGURE 2. Likewise the connections frommotor 90 to the static error corrector 52 including elements 427 through442 may correspond to the mechanical connection 112 in FIGURE 2.

Because cam 435 is characterized to convert log P to P motor 90 iscaused to turn as a function of log P Gear trains 429 and 432 operate toreduce the number of turns of motor 90 to a value compatible with thelimited amount of rotation available with cam 435. Since motor 90operates as a function of log P mechanical connections 427, 431 and 433also operate as functions of log P However, mechanical connections 440,442 and 444 operate as a function of P itself since they are on the farside of cam 435. Therefore, as described in connection with FIGURE 2, aninput on mechanical connection 442 indicative of P is presented to thestatic error corrector 52 and a rebalance signal indicative of P ispresented to transducer 50 by mechanical connection 444.

It has been found that the stability of the above described servo loopchanges as a function of static pressure so as to tend to be overdampedat low pressures. To compensate for this change in mechanical gain, amechanical connection shown :as dash line 450 is connected between geartrain 432 and the wiper 425 associated with the rate feedback networkabove described. Mechanical connection 450 turns as a function of log Pas above described and operates to position wiper 425 so that variousamounts of rate feedback damping are provided depending upon themagnitude of the static pressure. By providing more or less ratefeedback damping signal in accordance with the magnitude of staticpressure, servo loop damping compensation for the change of stability inthe servo loop with change of static pressure is accomplished thuspreserving optimum frequency response.

Also shown in FIGURE 3 is the q transducer 300 having a P input fedthereto by means of conduit 304 and a P input fed thereto by means ofconduit 303. A mechanical connection from the static error corrector 52is provided for the q transducer 300 as shown by dash lines 460 and 405.The mechanical connections 405 and 460 may comprise the roller 192, rack194, gears 196, 312 and 310 and the shaft 308 of FIGURE 2.

An electrical output from the q transducer is provided on the conduct-or462 which is shown connected to a summing network 463 and then by aconductor 464 to an amplifier 465. Amplifier 465 is connected to a motor467 by a connection 468. Rate feedback for this motor is provided bymeans of a velocity generator 469 connected to motor 467 by a connectionshown as dash line 470. The output of velocity generator 469 ispresented to a resistor 472 by a conductor 473. Resistor 472 has a wiper475 connected back to the summing network 463 to provide the requisiterate feedback. The output of motor 467 is shown as a dash line 480connected to a gear train 482.

Rebalance of the q transducer is provided by a connection shown as dashline 483 from gear train 482 to a second gear train 485. Gear train 485is connected to a characterized cam 487 by a connection shown as dashline 488. Cam 487 may be a tape cam like cam 435 and has a first portion489 and a second portion 490. The characterization of cam 487 is suchthat as cam 489 rotates as a function of log q portion 490 will rotateas a function of q Cam 487 is connected to q transducer 300 bymechanical connection shown as dash line 492, a gear train 493 and amechanical connection shown as dash line 495. Because of thecharacterization of cam 487, motor 467, mechanical connection 480,mechanical connection 483 and mechanical connection 488 move asfunctions of log q. whereas mechanical connections 492 and 495 move asfunctions of q Connection 495 to q transducer 300 provides rebalance inmuch the same fashion as was described with regard to the P transducer50 in FIGURE 2. Gear trains 482, 485 and 493 are used for purposes ofconverting the amounts of rotation of the shafts to smaller or largerquantities as desired.

As with the P servo loop, the q servo loop also changes gain inaccordance with changes of q A similar means of compensating for thischange of gain is shown in FIGURE 3 as a mechanical connection shown asdash line 496 connected between gear train 485 and wiper 475 of resistor472. As q changes, mechanical connection 496 will move wiper 475 toprovide more or less rate feedback and to compensate for the change ingain.

As previously stated the output of motor is operating at the function oflog P and the output of motor 467 is operating as a function of log qGear train 429 has a connection shown as dash line 501 connected to adifferential 503. This connection provides an input to the dilferential503 which is a function of log P Gear train 482 has a mechanicalconnection shown as dash line 505 connected to differential 503. Thisprovides an input to differential 503 which is a function of log q Theoutput of the differential is the algebraic difference between the twoinputs and this output is shown as dash line 510 in FIGURE 3. The signalon the output connection 510 may be expressed as log ry -log P =log ghas long been recognized as a function of air speed in terms of Machnumber and is usually written as R1 where R is defined as P /P Themotion of mechanical connection 510 is therefore a function of log (Rl)which in turn is a function of Mach number. Mechanical connection 510 isshown connected to a gear train 512 and then by a mechanical connectionshown as dash line 513 to a cam 515. Cam 515 is shown as a tape cam butunlike cams 435 and 487 is characterized to convert the function log(Rl) to a function of Mach number M. That is to say, as a first portion517 of cam 515 rotates as a function of log (R-l), a second portion 519of cam 515 moves as a function of Mach number M. A mechanical connectionshown as dash line 520 connected to cam 515 therefore rotates as afunction of Mach number M. Connected to mechanical connection 520 isanother mechanical connection shown as dash line 525 which is shownleading to a cam and follower arrangement similar to that shown inFIGURE 2 as cam 174 and follower arm 172. The purpose of this camarrangement is to provide an output indicative of the quantity AP/Pwhich, as previously explained, is a function of Mach number. Thismotion is presented to the static error corrector 52 by means of amechanical connection shown as dash line 527. Mechanical connection 525may be equivalent to the shaft 180 shown in FIGURE 2 while mechanicalconnection 527 may be equivalent to the pivot 166 in FIGURE 2. It shouldbe remembered at this point that log (R-l) is in itself a function ofMach number and if desired connection 525 could 'be connected tomechanical connection 510 rather than mechanical connection 520providing cam arrangement 174, 172 were characterized so as to convertlog (R1) to a function of AP/P As shown with regard to FIGURE 2 thestatic error correction mechanism 52 operating on an input of P and aninput of AP/P produces the required correction signal AP which is shownin FIGURE 3 as emerging on mechanical connection 405 and is presented tothe P transducer 50 and the q transducer 300 by mechanical connections402 and 460 respectively to provide the requisite correction for staticpressure error.

In order to produce a number of outputs necessary for aircraft operationthe air data computer as shown in FIGURE 3 has a variety of connectionsto various portions within the computer each operable to produce adesired output. As examples of the kinds of outputs frequently desiredin air data computers FIGURE 3 shows 11 terminals numbered 550 through560. Terminal 550 is shown producing an output d log P /d or rate ofchange of log P This output is a known function of rate of change ofaltitude li Rate of change of log P is easily derived in the circuit ofFIGURE 3 by a connection shown as conductor 562 connected to the outputconductor 424 of velocity generator 420. As previously stated motor 90is turning as a function of log P and therefore velocity generator 420produces an output indicative of rate of change of log P Standardcharacterized means may be employed to convert the signal d log P /dtoli Terminal 551 is shown having an output of log P This output isderived by a mechanical connection shown as dash line 564 connected to amechanical connection 450 which as previously mentioned moves as afunction of log P Of course, mechanical connection -564 could beconnected elsewhere in this system and still obtain a signal indicativeof log P For example, an output from gear train 429, mechanicalconnection 431 or mechanical connection 433 could be connected tomechanical connection 564 since each of these connections is moving as afunction of log P Terminal 552 is shown producing an output P This isderived in FIGURE 3 by mechanical connection shown as dash line 556connected to mechanical connection 442 which as previously mentionedmoves as a function of P As before, mechanical connection 566 could beconnected elsewhere in the circuit and still obtain a P output. Forexample connection 566 could be connected to connection 444 or geartrain 443 as easily.

Terminal 553 is shown producing an output A log P or change of log Pfrom some predetermined value. The change of log P output is a knownfunction of change of altitude h and may be used as an altitude holdsignal for the aircrafts autopilot. This altitude hold signal is derivedfrom a connection shown as dash line 568 which is connected to oneportion of a clutch 569, the other portion of which 570 is connected togear train 429 by a connection shown as dash line 571. Clutch 570 may beoperated electromagnetically by means of a coil 572. As previouslystated the outputs from gear train 429 operate as functions of log P andso connection 571 also move as a function of log P Since log P is afunction of altitude h, when altitude hold is desired the coil 572 maybeenergized by the pilot thereby engaging clutch members 569 and 570. Fromthe time of engagement of clutch, a signal is presented to outputterminal 553 which will show change of log P from that which existed atthe time of engagement. This change of log P may be used thereafter tocontrol the autopilot and hold the aircraft at the predetermined desiredaltitude. Clutch member 569 may be connected to a recentering spring notshown to bring clutch member 569 back to its initial position when theclutch is again deenergized.

Output terminal 554 is shown producing an output indicative of altitudeh This is derived from a mechanical connection shown as dash line 575connected to a cam 577. Cam 577 is connected by a mechanical connectionshown as dash line 578 to the mechanical connection 433 which aspreviously stated is moving as a function of log P Cam 577 is socharacterized as to convert log P to altitude h Output terminal 555 isshown producing an output of indicated air speed V This output isobtained by a mechanical connection shown as dash line 580 connected toa cam 582. Cam 582 is connected by a mechanical connection shown as dashline 583 to mechanical connection 488. As previously stated mechanicalconnection 488 is moving as a function of log q and hence connection 583moves as a function of log q Cam 582 is so characterized as to convertlog q to indicated air speed V Output terminal 556 is shown producing aMach number output M. This is derived from a mechanical connection shownas dash line 585 connected to mechanical connection 520 which aspreviously described moves as afunction of the Mach number.

Output terminal 557 is shown producing an output indicative of change oflog (R-l). This in turn is a function of change in Mach number and maybe used to produce a signal for the autopilot to provide a Mach holdfunction. The output of terminal 557 is derived from a mechanicalconnection shown as dash line 588 which is shown connected to oneportion of a clutch 589, the other portion 590 of which is connected bya mechanical connection shown as dash line 591 to mechanical connection510. As previously stated mechanical connection 510 moves as a functionof log (Rl) so that mechanical connection 591 similarly moves. Also aspreviously stated log (R-l) is a function of Mach number. Clutch members589 and 590 are engaged by a coil 592 so that when it is desired to holdMach, the pilot may engage members 589 and 590 after which mechanicalconnection 588 moves with mechanical connection 591. A signal is thusprovided indicative of change of Mach number from the value existing atthe time of engagement. This signal may be sent to the autopilot toprovide the desired Mach hold function. As with clutch 569, 570, clutch589, 590 may be recentered by means of a spring not shown so that whenthe clutch is deenergized member 589 will return to its originalposition.

Output terminal 558 is shown producing a signal indicative of q Thissignal is obtained from a mechanical connection shown as dash line 593connected to mechanical connection 492. As previously stated connection492 moves as a function of 11 so that shaft 593 moves likewise.

Output terminal 559 is shown producing a signal indicative of log q Thissignal is derived from a mechanical connection shown as dash line 595connected to the mechanical connection 496. As previously statedconnection 496 moves as a function of log q and hence connection 595does likewise. K

11 Output 560 is shown producing a signal indicative of d log (R1) r orrate of change of log (R1). Rate of change of log (R-l) is proportionalto rate of change of Maach and hence the signal at output 560 may beused to be indicative of Mach rate. This signal is derived from aconductor 600 shown connected to a summing apparatus 602 which may be astandard summing amplifier. Summing apparatus 602 is connected to theoutput of velocity generator 469 by a conductor 604 and is connected tothe output of velocity generator 420 by conductor 562 and a conductor606. A signal from velocity generator 469 is indicative of a rate ofchange of q whereas the signal from velocity generator 420 is indicativeof rate of change of P When combined in summing network 602 an output isobtained which is indicative of rate of change of q /P which aspreviously defined is a function of rate of change of Mach.

As stated previously with regard to the outputs log P and P theconnections leading to the other outputs can be connected elsewhere inthe circuit. For example, q output on 558 could be connected bymechanical connection 593 to connection 495 rather than 492 and the logq output on terminal 559 could be connected to mechanical connections480, 483 or 488 rather than connection 496. I therefore do not intend tobe limited to the specific connections shown in FIGURE 3 since oneskilled in the art could place the connections to other appropriateplaces to take best advantage of gear train scale factors.

The various shaft outputs 550 through 569 in FIGURE 3 may be connectedto data transmitting devices such as synchros or potentiometers tosupply signals to indicators and autopilot components in a standardstraightforward manner bearing in mind that several of the outputs wouldnecessarily require some characterization before final usage. Forexample with regard to the output at terminal 550 which was stated asvarying with the rate of change of log P when it is desired to providean indication or an autopilot signal indicative of altitude rate thissignal must be characterized to convert log P to h Likewise the altitudehold and Mach hold signals appearing on terminals 553 and 557 are onlyfunctions of these conditions and apparatus such as characterizedpotentiometers may be necessary to change A log P to M1,, and A log (R1)to AM. Such characterization is a straightforward state of the artprocedure and will not be described in detail here.

It is thus seen that apparatus has been provided which produces amechanical output indicative of corrected static pressure P and that anair data computer incorporating this apparatus has been provided whichsupplies a number of useful outputs to be utilized by the aircraft.Also, since static error correction is accomplished at the pressuretransducers, all outputs are compensated for the error in indicatedstatic pressure. It is further seen that the inherent disadvantages inelectrical and pneumatic systems have been overcome by means of thisnovel structure and that the apparatus provided is simple and easilymanufactured and assembled. Furthermore it is noted that the variouspressure transducers identified as the static pressure transducer q haveidentically the same form and may be constructed in the same mannerthereby reducing manufacturing costs. Likewise the static errorcorrection mechanism provides an output which may be utilized by both ofthe pressure transducers rather than having a separate corrector foreach transducer. This feature again simplifies the manufacturingprocess. Another advantage is seen in the transducer itself wherein theelements are so arranged around the pivot axis that symmetry isachieved. This is to say the Z-shaped member and the bellows arearranged on opposite sides of the pivot axis so as to minimize anyvibration and acceleration effects that might be produced in the systemwhen it is mounted in an aircraft.

Finally it should be noted that many modifications may be made to thestructure herein described without departing from the spirit of theinvention. For example, it is within the skill of one in the art tomodify the various shapes and connections herein in many obvious ways.

I therefore do not intend to be limited by the specific elementsdescribed in connection with the preferred embodiment but intend only tobe limited by the following claims.

What is claimed is:

1. Static error correcting apparatus comprising in combination:

a first member;

means positioning said first member in accordance with indicated staticpressure (P first means responsive to the position of said first memberand having an output movable in accordance therewith;

computing apparatus having a first input connected to the output of saidfirst means, and having a second input; function generating meansmovable in accordance with a function of airspeed in terms of Machnumber means connecting the second input of said computing apparatus tosaid function generating means, said computing apparatus operating toproduce an output movable in accordance with the desiredcorrectionfactor (AP);

means connecting the output of said computing apparatus to said firstmember to further position said first member in accordance with AP, theposition of said first member being indicative of the algebraic sum of Pand AP and thus indicative of corrected static pressure (P and outputmeans responsive to the position of said first member and operable toreposition said first member to accomplish rebalance. 2. Apparatus forcorrecting indicated static pressure as a predetermined function of Machnumber comprising, in combination:

pressure transducing means including a member movable in accordance withindicated static pressure;

responsive means generating an output signal in accordance with movementof the member of said pressure transducing means; correction meanshaving a first input connected to receive the output of said responsivemeans, a second input connected to receive a signal indicative of apredetermined function of Mach number, and producing an output forcewhich varies as the product of the signals at the first and secondinputs; and

means connecting said correction means to said pressure transducingmeans so that the output force is applied to the member of said pressuretransducing means, movement of the member being indicative of correctedstatic pressure. 3. In an air data computer having a source of indicatedstatic pressure P a pressure transducer having a first input connectedto the source of P having a second input for receiving a signalindicative of static pressure error AP, having a rebalance input andoperable to produce an output signal indicative of change of correctedstatic pressure P motor means connected to said pressure transducer andoperable to produce a signal indicative of log P characterized meansconnected to said motor means and operable to convert the signalindicative of log P to a signal indicative of P means connecting saidcharacterized means to the rebalance input of said pressure transducerto apply the signal indicative of P for purposes of rebalancing saidpressure transducer; and

13 14 means producing an output signal indicative of AP 3,090,229 5/1963Howard 73-182 connected to the second input of said pressure trans-3,108,183 10/ 1963 Ganley et a1. 73-1 82 X ducer. 3,132,244 5/1964Kemmer et a1. 73-182 X References Cited UNITED STATES PATENTS 5 LOUIS R.PRINCE, Primary Examiner. 2,736,199 2/1956 Ibbott 73 407 X DAVIDSCHONBERG: Examiner- 3,086,702 4/1963 Bowditch 23561 D. O. WOODIEL,Assistant Examiner.

2. APPARATUS FOR CORRECTING INDICATED STATIC PRESSURE AS A PREDETERMINEDFUNCTION OF MACH NUMBER COMPRISING, IN COMBINATION: PRESSURE TRANSDUCINGMEANS INLCUDING A MEMBER MOVABLE IN ACCORDANCE WITH INDICATED STATICPRESSURE; RESPONSIVE MEANS GENERATING AN OUTPUT SIGNAL IN ACCORDANCEWITH MOVEMENT OF THE MEMBER OF SAID PRESSURE TRANSDUCING MEANS;CORRECTION MEANS HAVING FIRST INPUT CONNECTED TO RECEIVE THE OUTPUT OFSAID RESPONSIVE MEANS, A SECOND INPUT CONNECTED TO RECEIVE A SIGNALINDICATIVE OF A PREDETERMINED FUNCTION OF MACH NUMBER, AND PRODUCING ANOUTPUT FORCE WHICH VARIES AS THE PRODUCT OF THE SIGNALS AT THE FIRST ANDSECOND INPUTS; AND MEANS CONNECTING SAID CORRECTION MEANS TO SAIDPRESSURE TRANSDUCING MEANS SO THAT THE OUTPUT FORCE IS APPLIED TO THEMEMBER OF SAID PRESSURE TRANSDUCING MEANS, MOVEMENT OF THE MEMBER BEINGINDICATIVE OF CORRECTED STATIC PRESSURE.